1. Technical Field
The invention generally relates to sound processing systems. More particularly, the invention relates to sound processing systems that configure audio signals to drive loudspeakers in a vehicle to maximize the frequency range of the audio output.
2. Related Art
Audio or sound system designs involve the consideration of many different factors. The position and number of speakers, the frequency response of each speaker, and other factors usually are considered in the design. Some factors may be more pronounced in the design than others in various applications such as inside a vehicle. For example, the desired frequency response of a speaker located on an instrument panel of a vehicle usually is different from the desired frequency response of a speaker located in the lower portion of a rear door panel. Other factors also may be more pronounced.
Consumer expectations of sound quality are increasing. In some applications, such as inside a vehicle, consumer expectations of sound quality have increased dramatically over the last decade. Consumers now expect high quality sound systems in their vehicles. The number of potential audio sources has increased also to include radios (AM, FM, and satellite), compact discs (CD) and their derivatives, digital video discs (DVD) and their derivatives, super audio compact discs (SACD) and their derivatives, tape players, and the like. Also, the audio quality of these components is an important feature. Moreover, many vehicle audio systems employ advanced signal processing techniques to customize the listening environment. Some vehicle audio systems incorporate audio or sound processing that is similar to surround sound systems offered in home theater systems.
Many digital sound processing formats support direct encoding and playback of five or more discrete channels. However, most recorded material is provided in traditional two-channel stereo mode. Matrix sound processors synthesize four or more output signals from a pair of input signals—generally left and right. Many systems have five channels—center, left-front, right-front, left-surround, and right-surround. Some systems have seven or more channels—center, left-front, right-front, left-side, right-side, left-rear, and right-rear. Other outputs, such as a separate subwoofer channel, may also be included.
In general, matrix decoders mathematically describe or represent various combinations of input audio signals in an N×2 or other matrix, where N is the number of desired outputs. The matrix usually includes 2N matrix coefficients that define the proportion of the left and/or right input audio signals for a particular output signal. Typically, these surround sound processors can transform M input channels into N output channels using an M×N matrix of coefficients.
Many audio environments, such as the listening environment inside a vehicle, are significantly different from a home theater environment. Most home theater systems are not designed to operate with the added complexities inside of a vehicle. The complexities include the complexity of outside sounds, such as road noise, wind noise, etc. In addition, vehicle listening environments may have non-optimal loudspeaker placement coupled with loudspeakers with various frequency response ranges. A vehicle and similar environments are typically more confined than rooms containing home theatre systems. The loudspeakers in a vehicle usually are in closer proximity to the listener. Typically, there is less control over loudspeaker placement in relation to the listener as compared to a home theater or similar environment where it is relatively easy to place each loudspeaker the same approximate distance from the listeners.
In contrast, it is nearly impossible in a vehicle to place each loudspeaker the same distance from the listeners when one considers the front and rear seating positions and their close proximity to the doors, as well as the kick-panels, dash, pillars, and other interior vehicle surfaces that could contain the loudspeakers. These placement restrictions are problematic considering the short distances available in an automobile for sound to disperse before reaching the listeners. In addition, the placement restrictions can also dictate the size and the optimal range of frequency response of the loudspeakers that are installed. Accordingly, a sound processing system is needed that can compensate for loudspeaker placement and provide signals to drive the loudspeakers within their respective ranges of frequency under varying operation conditions within a vehicle to optimize the frequency range of the audio output within the vehicle.